Figure 1. Coupling a Signal into a Circuit Path Using a Square Unshielded Wire Loop
(covered with heat shrink tubing)

Abstract: Shielded loops are
often used to minimize electric field (capacitive) coupling. A case is
shown where using a shielded loop to inject signals into a path on a circuit board
results in a significant resonance whereas using an unshielded wire
loop results in a relatively flat frequency response of the injected
signal. Unshielded wire loops are thus shown to be more useful than shielded loops in some
cases.

Discussion: Figure 1 shows a
square unshielded wire loop held up to a path crossing a break in the
ground plane of a test board that is used for many experiment on this
website. The injected signal was measured at the BNC connector on the
board (left side) for the
cases where the loop is positioned as shown and for a 180 degree
rotation of the loop and similarly for a square shielded loop (embedded
in plastic for strength) as shown in Figure 2. The construction of the
shielded loop is shown in the May 2008 Technical Tidbit, The Square Shielded Loop - Part 1.

Figure 2. Coupling a Signal into a Circuit Path Using a Square Shielded Loop
(embedded in a plastic housing for strength)

Figures 3 and 4 show the measured signal at the BNC connector on the
board for the
unshielded wire loop in the normal position (as in Figure 1) and for
the 180 degree rotated position of the loop respectively. The data was
taken using an Agilent N1996A spectrum analyzer as a two port insertion loss measurement.
The square loop was connected to the tracking generator output and the
BNC connector on the board was connected to the receiver input of the analyzer.

Capacitive coupling between the loop and the board will cause either a resonance effect (dip or peak
in the response) or a directional effect when the loop is rotated 180 degrees
because the phase of the inductive coupling changes by 180 degrees
whereas the capacitive coupling remains the same. As can be seen in Figures 3 and 4, there is no resonant effect, the
frequency response is nearly flat. The capacitive coupling itself is
very low compared to the inductive coupling because the difference
between Figures 3 and 4 is only a few dB and then only at the higher
frequencies above 600 MHz. Contrast the responses in Figures 3 and 4
for the unshielded loop to the responses in Figures 5 and 6 for the
shielded loop.

Figure 3. Injected Signal for Unshielded Loop

Figure 4. Injected Signal for Reversed Unshielded Loop

In both Figures 5 and 6, a resonant dip in the response is seen similar
to that shown for coupling between shielded loops in the June 2008 Technical Tidbit, The Square Shielded Loop - Part 2, Parasitic Coupling.
The reason for this resonance is described in that Technical Tidbit. In
this case, the resonance is due to the sum of the inductance of the
shields of the
loop and the inductance around the split in the ground plane
interacting with the capacitance between the shields and the
ground plane of the board. As one would expect for a shielded loop, the
plots in Figures 5 and 6 are not very sensitive to the normal and
rotated positions of the loop.

Figure 5. Injected Signal for Shielded Loop

Figure 6. Injected Signal for Reversed Shielded Loop

One can conclude from the above plots that the unshielded loop works
better for injecting signals into a path crossing a ground plane split
than does the shielded loop. I suspect this result holds in general for
injecting signals into circuit boards with ground and power planes.

Summary:Capacitive
coupling from an unshielded loop is not always a problem
that requires the use of shielded loops to solve. On the contrary,
sometimes unshielded loops work better than shielded loops. This series
of four
Technical Tidbits on square shielded loops has shown that unshielded
loops are useful for injecting signals in many cases and into circuit
boards specifically. Given the ease of constructing an unshielded loop
and its low cost, this is an important result. Another conclusion that
can be drawn is that shields are just thick wires with inductance
and capacitance and are not a "magic" solution to prevent unwanted
coupling in all cases.

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